Ionic liquids are organic matters that are liquid totally consisting of ions at room temperature or near room temperature. As a new electrolyte, ionic liquids exhibit a range of advantageous properties such as wide electrochemical potential window range, non-volatile, nonflammable and good thermal stability. Ionic liquids are promising electrolytes for double-layer capacitors. Traditional electrolytes are prone to decompose at high voltage, causing a sharp rise of internal resistance and a rapid fall of capacitance. Therefore stability of electrolyte is critical factor in specific energy of capacitor.